Problem: Solve for $r$ : $-21 = 28 + r$
Subtract $28$ from both sides: $ \begin{eqnarray} \\ \hphantom{-21 {- 28}} &\hphantom{=}& \hphantom{r} \\ -21 &=& 28 + r \\ \\ {-28} && {-28} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -21 {- 28} &=& r \\ \end{eqnarray} $ Simplify. $ r = -49$